Fuel burning engines, such as internal combustion engines, may receive fuel via a fuel system. Some fuel systems may deliver fuel to the engine via port fuel injectors, whereby fuel is injected into an intake port of the engine. Other fuel systems may deliver fuel to the engine via direct fuel injectors, whereby fuel is injected directly into the cylinders of the engine.
Other fuel systems have been proposed that include both port fuel injectors and direct fuel injectors. For example, the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. are one example. The Heywood et al. papers describe directly injecting ethanol via direct fuel injectors in order to improve charge cooling effects, while also relying on port injected gasoline to provide the majority of combusted fuel over a drive cycle. The ethanol, in this example, may provide increased octane and increased charge cooling at the engine due to its higher heat of vaporization as compared with gasoline. As such, the ethanol may be used to reduce knock limits that may be otherwise imposed on engine boosting and compression ratio. This approach purports to increase engine fuel economy and increase utilization of renewable fuels, such as ethanol, by enabling the use of greater engine boosting and compression ratio with reduced engine knock.
Some fuel systems have been developed for use with the above described multi-fuel engines, whereby a fuel mixture comprising a blend of two or more fuels may be separated into two or more fuel components on-board the vehicle via a fuel separator. Fuel separation may be improved or increased, with some fuel separators, in proportion to a pressure difference applied across the fuel separator. For example, a rate at which a fuel separator separates a fuel mixture into two or more fuel components may be increased by increasing the pressure at which the fuel mixture is supplied to the fuel separator. As such, some fuel systems may utilize a separate fuel pump to pressurize the fuel mixture and thereby achieve a prescribed fuel separation rate at the fuel separator.
However, the inventors herein have recognized a disadvantage with this approach. For example, even though the fuel pump may be operated to enable or improve the fuel separation process, this additional fuel pump may reduce fuel efficiency of the engine, increase the complexity of the fuel system, and increase the cost of the fuel system.
As one approach, these and other issues may be addressed by a fuel system and a method of operating the fuel system. As a non-limiting example, the method includes: varying a composition of fuel supplied to an inlet of a fuel pump responsive to engine output; operating the fuel pump to provide pressurized fuel at an outlet of the fuel pump using the fuel received at the inlet of the fuel pump; supplying the pressurized fuel from the outlet of the fuel pump to the internal combustion engine and to a fuel separator; and varying a proportion of the pressurized fuel supplied to the internal combustion engine relative to the fuel separator responsive to the engine output. As a non-limiting example, during a lower engine output range, the fuel pump may supply fuel having a lower heat of vaporization (e.g. a fuel having a lower alcohol concentration) to the fuel separator; and during a higher engine output range the fuel pump may supply fuel having a higher heat of vaporization (e.g. a fuel having a higher alcohol concentration) to the engine via a direct injector. The amount of the fuel having the higher heat of vaporization that is supplied to the engine via the direct injector may be varied relative to an amount of the lower heat of vaporization fuel supplied to the engine via a port fuel injector in response to engine operating conditions.
In this way, at a first operating condition (e.g. at lower engine outputs) a fuel pump may be operated to provide a fuel mixture to the fuel separator at a suitable pressure for facilitating fuel separation; while at a second operating condition (e.g. at higher engine outputs), the same fuel pump may be operated to provide a fuel having a higher heat of vaporization to the engine. This higher heat of vaporization fuel may have been previously separated from the fuel mixture during a previous lower output operation of the engine. This approach enables the fuel system to perform multiple functions with the same fuel pump, thereby enabling a reduction in cost and complexity of the fuel system, as well as improving fuel efficiency of the engine.